1. Field of the Invention
This invention relates to a drilling assembly and apparatus for balancing rotation of a drill bit or other portion of a drillstring and more particularly, but not by way of limitation, to a self-balancing apparatus for automatically compensating at least some imbalances which cause vibrations in a rotating drill bit.
2. Setting of the Invention
In drilling an oil or gas well, forces arise which can induce vibrations in the drill bit and/or other portions of a drillstring. These vibrations can cause early and catastrophic bit failure and reduced rates of penetration. The detrimental effects of vibrations have been found to be even more pronounced when operating at high rotational speeds, such as are obtained through the use of downhole drilling motors.
Such vibrations in a drill bit are also known as "backward whirl" or "bit whirl." This is the phenomenon wherein the drill bit spirals about the longitudinal axis of the borehole which is cut by the drill bit. When a drill bit vibrates or whirls, the cutting elements on the face of the bit travel at an average cutting speed which is greater than the speed at which the cutting elements would rotate on a true (i.e, non-whirling) drill bit. The cutting elements also travel sideways and backwards during whirl, and they can experience severe impact loads. As a result, the drill bit can wear out prematurely and the rate of penetration obtainable with such a bit can decrease. The wear and rate of penetration decrease can be catastrophic in that the drill bit can lose all of its cutting elements, and the drilling process will be stopped.
At high drilling speeds achieved through the use of drilling motors, such as conventional turbine or positive displacement motors, vibrations and bit whirl can be caused by a mass imbalance of the rotating assembly and/or drilling force imbalances; and/or if the entire drill string is rotating, it is another source of vibrations.
Mass imbalance is that imbalance force caused by an unbalanced distribution of mass on the rotating member (e.g., the drill bit). Although mass imbalance might be compensated for by carefully manufacturing the drill bit to insure a symmetrical distribution of mass, such static compensation techniques might be costly to manufacture or implement, would likely not be able to compensate during wearing of the drill bit which creates changes in the mass distribution, and would likely not compensate for the variable drilling force imbalance which is believed to be the main cause of harmful vibrations.
The cause of drilling force imbalance is described in copending U.S. patent application Ser. No. 133,684, filed Dec. 15, 1987, and assigned to the assignee of the present invention. Copending U.S. patent application Ser. No. 133,684 now U.S. Pat. No. 4,815,342 is incorporated herein by reference; however, a brief description of drilling force imbalance will be given herein.
When a drill bit is rotated, all of the forces normally applied to the bit act through the individual cutter elements disposed across the face of the bit. These forces can be resolved into an equivalent set of orthogonal forces and moments acting on the bit. The main moment is the torque required to spin the bit. The axial component of the orthogonal forces is the applied weight on the bit. The imbalance force arises if the nonaxial component of the contact forces between the rock and bit is not zero. This occurs when the vector sum of nonaxial force components of the forces acting on the cutting elements does not equal zero. This imbalance force will change with different operating conditions, with different formations, and as the bit wears. At any instant in time, however, the nonaxial imbalance force rotates with the bit at the speed of the bit. Because this imbalance force rotates in phase with the bit, the effect of the rotating imbalance force is additive to any mass imbalance.
In view of the problems which can arise due to mass imbalance and drilling force imbalance, there is the need for an apparatus and drilling assembly which automatically compensate for these imbalances to prevent vibrations from beginning. It is contemplated that accomplishing this would prevent or reduce premature bit failure and decreased rates of penetration arising from the described imbalances. Such apparatus and assembly should be able to make compensations automatically under dynamic conditions.